PROJECT SUMMARY/ABSTRACT: TARGETING DNA DAMAGE RESPONSE IN BREAST CANCER The increased genome instability caused by DNA repair defects can be exploited using synthetic lethal approaches. For example, poly-ADP-ribose polymerase (PARP) inhibitors including olaparib have been approved to treat breast and ovarian cancer patients harboring mutations in BRCA1 or BRCA2. These drugs are thought to work by targeting defects in homology-directed repair (HR) of double-strand breaks (DSBs). However, HR proteins, including BRCA1 and BRCA2, ?moonlight? to regulate replication fork stability, and there is increasing evidence that these replication fork functions can be important determinants of PARP inhibitor outcomes independently of HR. Importantly, PARP inhibitor resistance is an increasing clinical problem. Thus, defining clinically relevant resistance mechanisms, biomarkers of resistance, and ways to overcome or prevent resistance are important research goals. This proposal addresses these issues both at the mechanistic and patient levels. Our overall goal is to improve patient outcomes by defining the mechanisms by which HR proteins regulate replication fork stability, understanding the contribution of replication fork dynamics to PARP inhibitor sensitivity, and identifying drug combinations that can be used to overcome resistance. We will use cell culture and patient-derived xenograft models to define mechanisms that influence PARP inhibitor sensitivity, utilize patient-derived 3D-ex vivo organoid cultures to examine mechanisms of PARP inhibitor resistance and explore rational drug combinations that can overcome resistance mechanisms in clinical trials. Completing these studies will test the hypothesis that fork protection is an important determinant of therapeutic efficacy of PARP inhibitors, identify therapeutic strategies to overcome resistance and potentially provide a rationale for expansion of PARP inhibitor use for clinical investigation and treatment.